The so-called muon anomaly, first seen in an experiment at Brookhaven National Laboratory in 2001, hasn’t budged. For 20 years, this slight discrepancy between the calculated value of the muon’s magnetic moment and its experimentally determined one has lingered at a significance of about 3.7 sigma. That is a confidence level of 99.98 percent, or about a one-in-4,500 chance the discrepancy is a random fluctuation. With the just announced results from the Muon g-2 experiment at Fermi National Laboratory in Batavia, Ill., the significance has increased to 4.2 sigma. That is a confidence level of about 99.997 percent, or about a one-in-40,000 chance for the observed deviation to be a coincidence. By itself, the new Fermilab measurement has only 3.3 sigma significance, but because it reproduces the earlier finding from Brookhaven, the combined significance has risen to 4.2 sigma. Still, the latter falls short of particle physicists’ five-sigma discovery threshold.

The result has been long-awaited because of its potential to finally break the Standard Model of particle physics, a collection of the so far known fundamental constituents of matter that has been in place for about 50 years. This model presently contains a couple dozen particles, but most of them are unstable and therefore can’t be found just by looking at the matter that normally surrounds us. The unstable particles are, however, naturally produced in highly energetic events, such as when cosmic rays hit the upper atmosphere. They are also made in lab-created particle collisions, such as those used in the Fermilab experiments to measure the muon’s magnetic moment.

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